1) Field of the Invention
The invention relates to a bi-level control system for operating high intensity discharge lamps at a first light output level and a second, reduced light output level, and more particularly, to improvements in such a control system which employs a switched capacitor for regulating the power supplied to the lamps. The invention also relates to an HID lighting system having such a control system and to components of the control system.
2) Description of the Prior Art
High intensity discharge (HID) lamps include, for example, mercury vapor, metal halide, and high pressure sodium discharge lamps. These lamps are operated with a ballast circuit to control the lamp operating current because of the negative voltage-current characteristics of the discharge arc within these lamps. Conventionally, electromagnetic transformer ballasts having a series connected inductance and capacitance (L-C circuit) in the form of a choke and capacitor, have been employed for this purpose.
Typically, the ballast, HID lamp, and reflector are combined into a fixture, or luminaire. For general illumination of, for example, warehouses and factories, a large number of luminaires are suspended form a ceiling. Generally, a plurality of the luminaires are connected in an alternating current (AC) power supply branch circuit and controlled by a single switch or circuit breaker which is effective to switch all of the lamps between an "off" state, in which the lamps are completely extinguished, and an "on" state in which the lamps are operated at full rated power.
Recently, because of energy saving considerations, it has become desirable in other types of lighting systems, for example fluorescent lighting, to employ more sophisticated controls such as occupancy sensors to turn the lamps off when nobody is present in a room and to turn the lights on when someone enters. However, this is not practical for HID lamps, which typically require several minutes to ignite, warm-up and reach their full light output levels. Additionally, most HID lamps have hot re-strike problems which makes it difficult to re-ignite the lamp shortly after being turned off while they still remain at an elevated temperature. With some lamp-ballast combinations it may take up to approximately ten minutes after a lamp has been turned off before it will re-ignite. Thus, employing a control system which turns HID lamps completely off when someone leaves the lighted space is not feasible because the lamps will not provide sufficient light quickly enough if someone re-enters the space shortly thereafter.
However, if HID lamps are operated at a reduced power level, instead of being completely turned off, they will return to a full or near full output level within an acceptable period of time. U.S. Pat. No. 4,994,718 (Gordin) shows such a system and employs a conventional electromagnetic ballast having a series L-C circuit. The light output from the lamp is changed by switching the capacitance in series with the lamp between a first valve which provides a full light output level and a second, reduced valve which lowers the power to the lamp and provides a reduced, energy-saving light output level. For this purpose, a second, switched capacitor is provided in addition to the single capacitor normally employed in the ballast circuit. The two capacitors may be arranged in series or parallel with each other, with one of the capacitors being switched into or out of circuit with the other capacitor to change the capacitance of the L-C circuit.
Gordin's switch for switching the switched capacitor into and out of the ballast circuit is a single electro-mechanical slave relay provided in a ballast housing which contains ballasts and switched capacitors for several lamps. A second electro-mechanical control relay in a separate controller remote from the ballast housing is connected to the slave relay in each ballast housing. A mechanical toggle switch, switchable between a high and low position, controls the position of the control relay, which in turn controls the position of the slave relays to switch the switched capacitors into and out of the ballast circuit to change the light output of their respective lamps.
A disadvantage of the Gordin system, however, is that control of the slave relays is accomplished by a pair of power supply wires connected between the control relay and the slave relay, in addition to the pair of power supply wires used to supply power to the ballasts. Furthermore, one slave relay is used to control the switched capacitor of each of the several ballast circuits contained in a ballast housing. While a common relay might be favorable for the system in Gordin where several ballasts are disposed in a common housing, this is usually not the case. Rather, the more common arrangement is for each lamp and ballast to be in included in a luminaire, or fixture, several of which are spaced from each other in a ceiling. In this type of arrangement, a slave relay common to several luminaires is not practical because of the additional wiring needed to connect the slave relay and switched capacitors to each of the spaced luminaires. For the same reason, the use of an additional pair of power supply wires to connect the control and slave relays, separate from the power supply wires connecting the ballasts, is a disadvantage because of the extra wiring, which is costly in terms of materials as well as labor for installation. Additionally, the switch contacts of the electro-mechanical relay in Gordin are subject to damage from current surges by discharging of the switched capacitor when it is switched out of the L-C ballast circuit.
U.S. Pat. No. 4,931,701 (Carl) shows another bi-level control system which employs a switched capacitor. Instead of an electro-mechanical slave relay as in Gordin, a solid state zero-crossing relay is used. The zero-crossing relay is said to ensure that the switching-in or switching-out of the switched capacitor is timed to occur at a zero-crossing point of the applied voltage. This applies or removes the switched capacitor only when the voltage level is not able to cause excessive voltage spikes or surges by the switched capacitor if it is partly or fully charged when switched, which can cause damage to other components in the circuit.
A disadvantage of such a solid state relay is that it allows a small current flow to the switched capacitor when the relay is not specifically switched for dimming the lamp. The small current flow to the switched capacitor was found to cause unintentional dimming of the lamp from the full light output level. It has also been found that such a relay can false trigger and close at times other than zero-crossing of the input voltage to the lamp. Additionally, solid state relays are relatively expensive as compared to electro-mechanical relays. Furthermore, a pair of control input lines is connected directly to a pair of control inputs of the solid state relay. As in Gordin, the use of an additional pair of control wires connecting the relays is a distinct disadvantage.
As compared to more expensive systems that employ solid state ballast circuitry to provide variable dimming of HID lamps over a range of light levels, operation of HID lamps at only two or several discrete light levels by switching of a switched capacitor as in Gordin and Carl offers a cost effective alternative for achieving energy savings. However, it is desirable to improve upon these known implementations.